The present invention relates in general to negatively charged liquid developers and methods of using these liquid developers in electrostatographic imaging systems.
In an electrostatographic imaging process such as, for example, xerography, a xerographic plate containing a photoconductive insulating layer is imaged by uniformly electrostatically charging its surface followed by exposure to a pattern of activating electromagnetic radiation such as light to selectively dissipate the charge in illuminated areas of the photoconductive member to form an electrostatic latent image corresponding to the pattern of activating electromagnetic radiation. This electrostatic latent image may then be developed with a developer composition containing charged marking particles. The resulting marking particle image may, if desired, be transferred to a suitable receiving member such as paper.
Developer compositions may be in dry or liquid form. Conventional commercial liquid developers comprise a dispersion of pigments in a liquid hydrocarbon. Once the electrostatic latent image is formed on a photoconductive imaging member, it is transported through a bath of the liquid developer. When in contact with the liquid developers, the charged pigment particles in the liquid developer migrate to the electrostatic latent image and deposit thereon in conformance with the image. The photoconductive member may then be withdrawn from the liquid developer bath with the marking particles adhering to the electrostatic latent image in image configuration. A thin film of residual developer normally remains on the surface of the electrophotographic imaging member.
In their earliest applications, liquid developers took the form of pigment particles such as carbon black, which were dispersed in a petroleum distillate and had a charge applied thereto with a charge control agent such as a metal salt. The problem with the earliest liquid developers existed in their dispersion stability in that upon being stored for any extended period of time, the carbon black pigment would tend to flocculate and settle out of the dispersion medium as non-redispersable macroscopic material at the bottom of the vessel. In an attempt to overcome this difficulty, a dispersant such as polyisobutylene which is soluble in the carrier liquid and which would be adsorbed on the carbon black pigment particles, was added in an attempt to provide a steric barrier between the individual particles. In effect, this was an attempt to provide increase dispersion stability by increasing the repulsive interaction between the individual carbon black particles and to provide a more uniform dispersion so that the particles would not settle out. It was believed that the presence of the resin maintained the carbon black as discrete particles over long periods of time by providing a protective coating for the carbon black particles so that the attractive forces between adjacent particles would not come into play. While this was a dramatic improvement over the liquid developers without dispersants that had been used heretofore, the resin coating in some instances tended to desorb from the carbon black particles thereby permitting the attractive forces between adjacent particles to once again come into play. This resulted in individual carbon black particles flocculating and settling to the bottom of the dispersion vessel.
The next step in the evolution of the development of liquid developers involved the use of amphipathic copolymers. For example, instead of the polyisobutylene homopolymer dispersant described above which was soluble in most of the aliphatic hydrocarbons that were used as dispersion vehicles and which also coated the carbon black, an amphipathic block or graft copolymer was selected on the theory that part of the copolymer would have an affinity for the liquid phase, the hydrocarbon liquid, and part of the copolymer would have an affinity for the surface of individual pigment particles. Thus, with the use of such an amphipathic copolymer, part of the copolymer is adsorbed on the carbon black particle surface and binds the insoluble part of the polymer to the particle surface thereby reducing the desorption of the polymer from the carbon black particles. Typical approaches are described in U.K. Pat. No. 3,554,946 (Okuno et al), U.S. Pat. No. 3,623,986 (Machida et al) and U.S. Pat. No. 3,890,240 (Hockberg). Even with this improvement in liquid developers, dispersion stability continued to present a problem in that it was also possible that the stabilizer desorb from the particle surface rendering the developer thermodynamically unstable. The next event in the development of liquid developers involved an attempt to formulate a developer in which desorption of the dispersant was, in effect, theoretically impossible. It was believed that a stable liquid developer would be provided if the particle contained a steric barrier which could not be desorbed from the particle surface. This, of course, is very difficult to do in the chemical sense when one is dealing with a carbon black pigment. The way around this particular difficulty, however, is to chemically make a particle wherein the steric barrier is chemically tied to the particle surface. This is typically accomplished with a non-aqueous dispersion of polymer particles wherein a steric barrier is attached to the polymer surface thereby providing a thermodynamically stable polymer particle. This provides a liquid developer in which the individual marking particles do not flocculate.
The above-described non-aqueous dispersion of polymer particles with a steric barrier attached to the polymer surface is described in detail in U.S. Pat. No. 3,900,412 (Kosel) which is incorporated herein in its entirely. Briefly, Kosel shows the concept of chemically providing a stable developer by forming a polymer core with a steric barrier attached to the polymer surface. The problem that exists with the technique described by Kosel relates to providing a sufficient amount of colorant associated with the marking particle to achieve an acceptable optical density in the developed image. For example, beginning at column 15 of the Kosel patent, a discussion may be found pertaining to imparting color by either using pigments or dyes and physically dispersing them as by ball milling or high shear mixing. Attempts to impart color by ball milling pigments added to the latex were unsuccessful insofar as obtaining a developed image of acceptable optical density. This is because the preferred size of latex particles is 0.2 to 0.3 micrometer in diameter and, with ball milling techniques, it is very difficult to prepare a dispersion of carbon black or other pigment particles much smaller in size than about 0.7 to about 0.8 micrometer. Consequently, for example, the addition of carbon black pigment particles to the relatively small latex particles while ball milling would only result in the relatively small latex particles residing on the surface of the pigment particles. The resulting developer particles are thermodynamically unstable.
A discussion may be found in the Kosel patent regarding the use of dyes as distinguished from pigments in providing color to a liquid developer. While this techique does work to a certain degree, it is still not possible to incorporate sufficient dye in the particles to give an image of acceptable optical density. Furthermore, and more importantly, the use of this approach will increase the level of background deposits because all the dyes described in column 16 and indicated in the Kosel patent to be capable of use in this technique are soluble in the dispersion medium. Since, as described above, the liquid development technique involves substantially uniform contact of the imaging surface with the liquid developer, including the insulating liquid carrier fluid, this fluid must come in contact with the electrostatographic imaging surface and the dye can be readily adsorbed onto the electrophotographic imaging surface, particularly single use zinc oxide photoreceptors, giving rise to increased background deposits in the final copy.
In U.S. Pat. No. 4,476,210 (Croucher et al) a stable color liquid developer is describe comprising an insulating liquid dispersion medium having dispersed therein colored marking particles which comprise a thermoplastic resin core which is substantially insoluble in the dispersion medium, an amphipathic block or graft copolymer steric stabilizer which is chemically or physically anchored to the resin core and which is soluble in the dispersion medium, and a colored dye imbibed in the thermoplastic resin core, the colored dye being dispersable at the molecular level and therefore soluble in the thermoplastic resin core and insoluble in the dispersion medium. In a preferred application, the dispersion medium is an aliphatic hydrocarbon, the amphiphatic steric stabilizer is a graft copolymer of poly (2-ethylhexyl methacrylate) or poly (2-ethylhexyl acrylate) solution grafted with vinyl acetate, N-vinyl-2-pyrrolidone or ethyl acrylate and a thermoplastic resin core which is a homopolymer or copolymer of vinyl acetate, N-vinyl-2-pyrrolidone or ethyl acrylate. The entire disclosure of U.S. Pat. No. 4,476,210 is incorporated herein by reference. Although positive or negative charging of dyed particles is mentioned in column 10, lines 35 and 37, all the specific formulations described in U.S. Pat. No. 4,476,210 are positively charged ink formulations which use zirconium octoate as the preferred charge control agent. The ink formulations in U.S. Pat. No. 4,476,210 were found to charge positively using a large variety of well known charge control agents including metal soaps. These formulations were aimed primarily at electrographic printing applications where the latent image is created by discharge of metal stylii. In this technology negatively charged latent images have traditionally been favoured because historically it has been easier to obtain stable positively charged liquid development inks than negatively charged liquid development inks. More recent ion stream deposition techniques lay down a positively charged latent image rather than a negatively charged latent image because stable positively charging corona devices are more readily available and more reliable than negatively charging corona devices. Also chalcogenide based photoreceptors, including migration imaging members (XDM), provide for a positively charged latent image to be toned. This has led to a need for negatively charged liquid inks and numerous examples of negatively charged carbon black based inks can be found in the patent literature. No specific examples of acceptable negatively charged latex based inks have been described to date.
At the present time the mechanism of electrostatically charging particulate matter in dielectric fluid is poorly understood from a scientific viewpoint, consequently it remains an intuitive process. In the case of carbon black based liquid development inks the charging appears to be caused by the interaction of the charge control agent with specific surface chemical groups on the carbon black. In the case of latex based liquid development inks such as described in U.S. Pat. No. 4,476,210, the surface characteristics which are important to charging are complicated since there is a resin core with a dye imbibed within this resin. The interaction of a specific dye and the resin makes it impossible to predict the effect of a charge control agent a priori.